1. Field of the Invention
The invention herein relates to the stabilization of talc-filled polystyrene. More particularly, it relates to a method for reducing the short term color degradation of polystyrene caused by incorporation of talc into the polystyrene.
It is common practice to use small amounts (20% or less) of talc as a filler in polystyrene compositions. The polystyrene and talc are milled together (i.e., compounded) until the talc is thoroughly dispersed through the polystyrene. Also present in the polystyrene are certain chemical stabilizers, such as di-tert.-butyl-p-cresol ("DBPC"; also known as butylated hydroxy-toluene or "BHT"). The work of milling the talc and polystyrene causes shearing in the resin and an increase in the resin temperature. These serve to enhance the catalytic effect of the talc and the net result is a tendency to color degradation of the polystyrene. However, in normal practice, the presence of the stabilizer is sufficient to neutralize the catalytic effect of the small amounts of talc filler. It is desirable in many instances, however, to have a talc filler content considerably in excess of 20%. Talc filer contents of 30 to 50% are desirable for many products, and the filler content may even be as high as 70% when a polystyrene/talc concentrate is formed for later master batching. With these high talc contents (i.e., greater than 20%) the normal amounts of stabilizers are ineffective to prevent the color degradation caused by the talc. Addition of more stabilizer would be impractical in terms of cost and also because of deleterious effects which might be caused by the excessive amounts of conventional stabilizers.
It would therefore be desirable to have a way of stabilizing polystyrene such that the polystyrene could tolerate large talc filler contents without significant short term color degradation during milling.
2. Description of the Prior Art
It has been known in the past that talc has a deleterious effect on polyolefins; see U.K. Pat. No. 1,219,783. This reference shows the use of a polyhydric alcohol (such as pentaerythritol) to overcome the degrading effects of talc or asbestos in a polyolefin. U.S. Pat. No. 3,799,906 shows the degrading effect of asbestos on polypropylene and teaches the use of polyester forming reactants including an organic acid or anhydride and a polyhydric alcohol mixed with the asbestos and polypropylene at an elevated temperature to provide a heat-stabilized, asbestos-filled polypropylene composition. U.S. Pat. No. 3,553,158 describes the degrading effect of talc on polyolefins and teaches the use of "talc deactivators", including aliphatic polyols as stabilizers. Several other references related to polyolefin degradation are cited in the aforementioned U.S. Pat. No. 3,799,906.